Myth busters
- Thread starter DianaOfBurlington
- Start date
Just a dark spot that has been there since the Lord came over on the Mayflower, I hardly notice it anymore.Did somebody lose a bet and shoot the chart table?Jeff
The 'max speed' was what I was referring to..
Did you hook up a drill to the little paddle wheel? Hahhh "Starts with one or two Pulls"Just a dark spot that has been there since the Lord came over on the Mayflower, I hardly notice it anymore.
The 'max speed' was what I was referring to..
All U Get
It actually IS mysterious. Really, I couldn't tell you why that says 19 knots, because I certainly do not remember it..
That is not entirely true.
GPS subsystems calculate speed over ground (SOG) by one of two methods.
1) Distance delta between two location readings take one second apart
2) Measuring actual doppler shift at the time of a reading.
Both mechanism can generate spurious readings due to signal multi-path, and/or rapid short moments near the antenna at the time of the reading. When this happens, the speed reported to the device is NOT the correct speed. But many devices simply report this anyway, because in the bigger picture it does really do not factor.
But for devices that care to report accurately (like top-end sail racing gear), there are steps they can take.
1) Have a max G acceleration factor built in. Boats cannot be going 4.4 knots one second, and 19 the next. Or even 7 then 19. If you know fast a boat/car/etc can accelerate, you know how much the speed can change reading to reading. Throw that point out.
2) Run an average of N points to report high speed. Simple but not as good as #1
3) Report an SUSTAINED high speed. Like for 10 or 20 seconds.
Here is a Velocitek ProStart, showing a top burst speed of 11.2 knots,controlled by rule #1, and a sustained top speed of 10.1 for 20 seconds.
Thats real.
Interesting. Is your vehicle equipped with a GPS transponder device that you're unaware of?
Maybe look in the fenders, wheel wells or on the undercarriage?
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how much was the fine it may have been cheaper to just go with the flow..... if you have no history of speeding then it would be your call ..as for clocking with a gps that is not an accurate way to tell example if two cars are going the same direction and the lead car is going 70 mph the second car always has to go faster to keep up ...cant explain why just know that is what happens
I have a Sony POV camera I use on the boat which has a built in GPS. Every so often it gets very confused and reports sustained higher speeds over a very extended period of time. Since it is also reporting position, It showed I tacked, when I didn't. This was probably over 400 feet off course. Then it showed I tacked back. all at about 18 mph. There may be something weird going on on that lake as the cell phone has also reported some sustained erroneous speeds in same area.
I would never trust any GPS that had a SONY label on it. It's not their core business, and they will never sweat the details.
Many factors in multi-path will create errors. There are tricks than can help sort them, better design helps, as does advanced technology like WAAS. Here is an interesting picture of 6 different GPS modules walking around a building. The true track in thin yellow. The wild tracks and quick returns to close approximate true position will generate huge spikes in reported speed. If you don't deal with that... hey 19 knots! ;^)
